Technical Field
The present disclosure relates to a wafer level package for a microelectromechanical (MEMS) sensor device and to a corresponding manufacturing process.
Description of the Related Art
As it is known, current packages for MEMS sensor devices, such as accelerometers, gyroscopes, magnetometers, pressure or force sensors, follow a standard process flow of die-attach of dies to a substrate, wire bonding and encapsulation.
FIG. 1 shows an exemplary MEMS sensor device 1, with an LGA (Land Grid Array) package 2.
The MEMS sensor device 1 includes a first die 3, including semiconductor material, e.g., silicon, and including a structural layer 3′ and an active layer 3″, wherein a micromechanical sensing structure S is integrated and includes, for example, a membrane suspended over a cavity, an inertial mass, elastic elements and/or other micromechanical sensing parts.
First die 3 has a front surface 3a, defined by the active layer 3″, at which the micromechanical sensing structure S is formed, and a back surface 3b, defined by the structural layer 3′, opposite to the front surface 3a with respect to a vertical direction z (the first die 3 having a main extension in a horizontal plane xy, orthogonal to the vertical direction z). First die 3 may also integrate further mechanical or electronic components, depending on the applications.
The MEMS sensor device 1 also includes a second die 4, including semiconductor material, e.g., silicon, and including a respective structural layer 4′ and a respective active layer 4″, wherein an electronic circuit A (so called ASIC—Application Specific Integrated Circuit), is integrated, shown schematically and operatively coupled to the micromechanical sensing structure S, e.g., to process electrical signals generated in response to detected quantities (such as linear or angular accelerations, pressures or forces) and to provide processed output signals outside of the package 2.
Second die 4 has a respective front surface 4a, defined by the active layer 4″, at which the ASIC circuit A is formed, and a back surface 4b, defined by the structural layer 4′, opposite to the front surface 4a, with respect to vertical direction z.
The first and second dies 3, 4 are stacked in the vertical direction z, i.e., the first die 3 is arranged on the second die 4, with the back surface 3b of the first die attached to the front surface 4a of the second die 4, with the interposition of an adhesive layer 5 (or adhesive layers, as shown in the FIG. 1).
In the example, the second die 4 has a horizontal extension (in the horizontal plane xy, orthogonal to vertical direction z), that is larger than a corresponding horizontal extension of the first die 3.
Electrical connections between the first and second dies 3, 4 are made through wire bonding, with electrical wires 6 connecting first pads 7 carried by the front surface 3a of the first die 3 to second pads 8 carried by the front surface 4a of the second die 4 (arranged where the same front surface 4a of the second die 4 is not covered by the first die 3). In particular, the first pads 7 are electrically coupled to the micromechanical sensing structure S, while the second pads 8 are electrically coupled to the ASIC circuit A.
The MEMS sensor device 1 further includes a substrate 9, e.g., a multi-layered substrate includes stacked conductive and dielectric layers, which acts as a base and bottom external surface for the package 2.
The stack of the first and second dies 3, 4 is arranged on the substrate 9; in particular, the back surface 4b of the second die 4 is attached to a front surface 9a of the substrate 9 via a further adhesive layer 11 (or adhesive layers, as shown in FIG. 1).
Further electrical wires 12 connect third pads 13 carried by the front surface 4a of the second die 4 (and electrically coupled to the ASIC circuit A) to fourth pads 14 carried by the front surface 9a of the substrate 9 (arranged where the same front surface 9a is not covered by the stack of the first and second dies 3, 4).
A back surface 9b of the substrate 9 faces the outside of the package 2, and carries external connections to external devices, e.g., for soldering to an external printed circuit board (PCB) of an electronic apparatus (not shown), in which the MEMS sensor device 1 is integrated. In particular, the back surface 9b of the substrate 9 carries electrical connection elements, in the example in the form of conductive lands 15, and further electrical connections 15′ are provided through the substrate 9 (so called TSV—Through Silicon Vias), for connecting the same conductive lands 15 to the fourth pads 14.
Other known solutions may envisage use of balls or spheres for electrical connection to an external printed circuit board (PCB); these packages are known as BGA, Ball Grid Array packages.
The MEMS sensor device 1 moreover includes a mold compound 16, e.g., of an insulating resin material, which covers and surrounds the stack of the first and second dies 3, 4 and moreover covers the front surface 9a of the substrate 9 (where the same front surface 9a is not covered by the stack of the first and second dies 3, 4). The electrical wires 6, 12 are embedded within the mold compound 16.
A front surface of the same mold compound defines a top external surface of package 2 of MEMS sensor device 1.
This standard package assembly, although advantageous in many respects, suffers from some drawbacks.
In particular, the package 2 has a dimension (especially in the vertical direction z) that may not be compatible with many applications, where size is an important design parameter, e.g., in portable or wearable electronic devices.
Moreover, the electrical wires 6, 12 may be subject to breaking during the molding process, this leading to failure of the manufactured MEMS sensor device 1.
In order to address these issues, some solutions have already been proposed, envisaging elimination of the substrate 9 (the so called wafer-level package), or the electrical connection between the first and second dies 3, 4 with the flip-chip technique, for achieving die bonding together with electrical connection.
However, an altogether satisfactory packaging solution for a MEMS sensor device, having reduced size (e.g., in the vertical direction) and desired mechanical and electrical performances continues to be desired.
In particular, important issues that remain outstanding are how to provide electrical connections to the outside of the package, e.g., for soldering to an external printed circuit board, without resorting to the use of complex and expensive manufacturing process steps.